And I'm Audie Cornish. Has the so-called red line been crossed in Syria? Today, the Obama administration said it believes the Syrian regime has used chemical weapons and, as President Obama has said in the past, that is a red line that would trigger serious consequences. But as NPR's Mara Liasson reports, the administration says it still has to evaluate the evidence and decide what actions to take.

MARA LIASSON, BYLINE: Just two days ago, the White House said it had not come to a conclusion about Syria's use of chemical weapons. Today, that conclusion and the increased involvement in the Syrian civil war that would follow is getting harder to avoid. In a letter sent to Senators Carl Levin and John McCain, the White House said the intelligence community believes with, quote, "varying levels of confidence" that chemical weapons have been used on a small scale.

Today, Defense Secretary Chuck Hagel spoke to reporters in Abu Dhabi.

CHUCK HAGEL: We cannot confirm the origin of these weapons, but we do believe that any use of chemical weapons in Syria would very likely have been originated with the Assad regime. As the letter states, the president has made it clear that the use of chemical weapons or the transfer of such weapons to terrorist groups would be unacceptable.

LIASSON: President Obama himself has laid down this marker repeatedly over the past year.

PRESIDENT BARACK OBAMA: We have put together a range of contingency plans. We have communicated in no uncertain terms with every player in the region that that's a red line for us and that there would be enormous consequences if we start seeing movement on the chemical weapons front or the use of chemical weapons. That would change my calculations significantly.

LIASSON: White House officials aren't ready to say the red line has been crossed. They say intelligence assessments alone are not enough and that they need credible and corroborated facts. But California Senator Dianne Feinstein, the chairman of the intelligence committee, said today the red line has been crossed. And Senator McCain, who's been pushing the White House to intervene more forcefully on behalf of the Syrian rebels, agreed.

SENATOR JOHN MCCAIN: The president of the United States said that if Bashar Assad used chemical weapons, it would be a game changer, that it would cross a red line. I think it's pretty obvious that red line has been crossed.

LIASSON: McCain has been calling on the administration to send weapons and provide a no-fly zone for the rebels. Former White House spokesman Tommy Vietor says he's certain the president will take some kind of action in response, but he has to decide what kind of escalation makes sense, understanding that every intervention carries a cost.

TOMMY VIETOR: If these allegations are true, there will be an effort to coordinate with the international community, but then, you know, there's obviously more difficult options like a no-fly zone or a humanitarian court order discussed. But I think, you know, your listeners should understand that those will involve a significant military action that will cost money and risk American lives.

LIASSON: Short of military intervention, the White House hopes this latest announcement will ratchet up the pressure not just on Bashar al-Assad, but on China and Russia, who've been blocking stronger international action against the Syrian regime. Mara Liasson, NPR News, the White House. Transcript provided by NPR, Copyright NPR.